Ion implanters are widely used in semiconductor manufacturing to selectively alter the conductivity of materials. In a typical ion implanter, ions generated from an ion source may be transported via an ion beam through a series of beamline components, which may include one or more analyzer and/or collimator magnets and a plurality of electrodes. The analyzer magnets may be used to filter out contaminant species or ions in the ion beam having undesirable energies. The collimator magnets may be used to manipulate the shape of the ion beam or otherwise adjust the quality of the ion beam before it reaches a target wafer. Suitably shaped electrodes may be used to modify the energy and the shape of the ion beam. After the ion beam has been transported through the series of beamline components, it may be directed to an end station to perform ion implantation.
FIG. 1 shows a known ion implanter 100 which comprises an ion source 102, extraction electrodes 104, an analyzer magnet 106, a first deceleration (D1) stage 108, a collimating magnet 110, and a second deceleration (D2) stage 112. The D1 and D2 deceleration stages (also known as “deceleration lenses”) are each comprised of multiple electrodes with a defined aperture to allow an ion beam to pass therethrough. By applying different combinations of voltage potentials to the multiple electrodes, the D1 and D2 deceleration lenses can manipulate ion energies and cause the ion beam to hit a target wafer 114 at a desired energy.
As the semiconductor industry keeps reducing feature sizes of micro-electronic devices, ion beams with lower energies are desirable in order to achieve shallow dopant profiles for forming shallow p-n junctions. Meanwhile, it is also desirable to maintain a relatively high beam current in order to achieve a reasonable production throughput. Such low-energy, high-current ion beams may be difficult to transport within typical ion implanters due to space charge blow-up. To prevent “blow-up” of an ion beam, oppositely charged particles, for example electrons or negative ions for positive ion beam, may be introduced for space charge neutralization. One way of sustaining space charge neutralization is through magnetic confinement of negatively charged particles, e.g., electrons. However, existing magnetic confinement approaches tend to introduce extra magnetic field components that can cause ion beam distortion unless carefully designed. In addition, the existing confinement schemes such as multi-cusp magnetic confinement, electrostatic confinement, and other confinement schemes, are not capable to diffuse the charged particles, especially electrons, along the beam direction, which is critical for effective beam neutralization. Moreover, in order to improve low-energy beam transportation, a high-energy ion beam may be decelerated to a desired energy level before reaching a target (e.g., a wafer). In such cases, some ions may go through “charge exchange” with surrounding neutral particles, thus losing their charge state prior to deceleration and generating neutral particles having high energy. Neutral particles having high energy fail to be decelerated and may impact the target at a higher energy level than desired, thus negatively impacting implantation results.
Low-energy ion beams may also be difficult to transport through the beamline to the target due to mutual repulsion between ions having the same charge. High-current ion beams typically include a high concentration of charged ions that tend to diverge due to mutual repulsion. To maintain low-energy, high-current ion beam quality, charged particles such as electrons or a plasma may be injected into the ion beam for the purpose of charge neutralization.
High-energy ion beams typically propagate through a weak plasma that is a byproduct of beam interactions with residual or background gas. This plasma tends to neutralize space charge caused by the ion beam, thereby largely eliminating transverse electric fields that would otherwise disperse the ion beam. However, for a low-energy ion beam, the probability of ionizing collisions with background gas is lower compared to a high-energy ion beam. In addition, low-energy ion beam blow-up may occur at much lower transverse electric field strength. The existing confinement schemes such as multi-cups magnetic confinement, electrostatic confinement, and other confinement schemes are not capable to diffuse the charged particles, especially electrons, along the beam direction due to the presence of dipole field. Therefore, the existing confinement schemes provide limited capability for an effective beam neutralization.
In view of the foregoing, it may be understood that there are significant problems and shortcomings associated with current techniques for transporting low-energy ion beams.